1. Field of the Invention
The present invention relates to a level including a telescope and a leveling device that is widely used in the construction fields and more specifically, to a level of the type having an autofocus system provided for the focusing optical system of the telescope through which a reference position of a subject is automatically brought into focus.
2. Description of the Related Art
A level is a piece of equipment generally used at a construction site by an engineer or surveyor for leveling, or for measuring bearings, horizontal angles, vertical angles, etc. The level is generally used with a tripod, the level being attached thereon.
One type of widely known level is the automatic level which is equipped with an automatic leveling instrument or device for automatically establishing a horizontal plane of sight.
The automatic level basically comprises a surveyor's telescope and a horizontal plane establishing optical system (horizontal plane compensating optical system) which functions as an automatic leveling device. The automatic level will now be explained.
When a reference position (sighting point that is set at a distance from the level), is sighted through the telescope, the horizontal plane establishing optical system ensures that a horizontal fine line of a reticle of the telescope lies on a real horizontal, even if the optical axis of the telescope is not correctly positioned in a horizontal plane. When another sighting point is sighted after the telescope has been rotated about the vertical axis, perpendicular to the optical axis thereof, the sighting point is located in the same horizontal plane as the reference position.
The optical system of the telescope of such an automatic level comprises an objective lens group, a focusing lens group and an eyepiece, arranged in this order from the object side. Due to the focusing lens group, a clear image of a sighted reference object (reference point) can be observed, regardless of the object distance. The position of the focusing lens group is adjusted depending on the object distance, so as to form a sharp object image on the reticle provided on the focal plane. The object image formed on the reticle can be viewed through the eyepiece.
Assuming that the observable object distance range of the surveyor's telescope is for example, 0.2 meters to .infin. (infinity), and that the focusing lens group is comprised of a concave lens, the movement range of the focusing lens group is approximately 30 mm. The focusing lens group is usually moved along the optical axis by rotating a rotatable focusing knob provided on the telescope. If the range of movement of the focusing lens group is small relative to the amount of rotation of the focusing knob, it is sometimes necessary to rotate the knob by a large amount to move the focusing lens group to a position at which an in-focus condition is obtained. That, it sometimes takes a long time to obtain an in-focus condition, although the image remains on the reticle on the focal plane for a long period of time.
Conversely, if the range of movement of the focusing lens group is large relative to the rotation of the focusing knob, achieves proper focusing. However the focusing knob need not be rotated by a large amount the time in which the image remain on the reticle on the focal plane is too short with respect to the amount of rotation of the focusing knob. That is, the focusing lens group moves by a large amount even when the focusing knob is rotated by a small amount. It is therefore difficult to obtain an in-focus condition quickly since the focusing knob must be rotated step by step, resulting in a time consuming operation.
Furthermore, it is sometimes the case in the conventional automatic level that an in-focus condition is obtained by a slight rotation of the focusing knob when the object to be sighted is located far away, whereas a large amount of rotation of the focusing knob is required to obtain an in-focus condition when the object is located at a close distance. Still furthermore, since it is impossible for the naked eye to check whether the object to be sighted is in a front or rear focus state, the focusing knob is often firstly mistakenly rotated in a wrong direction, i.e., opposite to the direction needed for focusing. In any event, in the conventional automatic level, the focusing operation is troublesome and requires an extended period of time.